Every once in a while an Apple patent application will surface in Europe that has bypassed the U.S. Patent Office for reasons unknown. Late yesterday Patently Apple discovered a patent application covering Apple Watch that specifically discusses a multi-modal physiological sensing system. In plain English, the patent is covering the Apple Watch Heart Rate Monitor. And, it looks as though Apple may be introducing a new method for compensating for motion so that users could perhaps one day jog and swing their arms without affecting the heart rate readings as they do today.
An Overview of Apple's Invention
A photoplethysmogram (PPG) signal may be obtained from a pulse oximeter, which employs a light emitter and a light sensor to measure the perfusion of blood to the skin of a user. However, the signal may be compromised by noise due to motion artifacts. That is, movement of the body of a user may cause the skin and vasculature to expand and contract, introducing noise to the signal. Further, the device itself may move with respect to the body of the user, introducing further noise.
To address the presence of motion artifacts, examples of the present disclosure can receive light information from each of two light guides, one in contact with the tissue of the user and one not in contact with the tissue of the user. First light information can be obtained from the first light guide, and second light information can be obtained from the second light guide. A heart rate signal can then be computed from the first and second light information, for example, by using blind source separation and/or cross-correlation.
Examples in this patent filing are directed to a system for sensing and measurement of physiological signals that enables a signal decomposition approach to physiological measurements. That is, multiple signals may be obtained, and each signal may contain a physiological signal of interest (e.g., a heart rate signal). Each signal may also contain different noise components, and thus signal decomposition methods may be used to filter the noise and compute the signal of interest.
Multiple sensing modalities optimized for the signal decomposition approach may provide robustness against nonsignal artifacts commonly induced in wearable sensors, such as motion- or biologically induced artifacts. Sensing modalities may include detection of physiological signals from optical sensors, force and pressure sensors, temperature sensors, accelerometers, proximity detectors, and/or impedance sensors, among other possibilities.
In some examples, sensing modalities may include optical sensors including light guides in contact and not in contact with tissue of a user. Because the signal of interest may be compromised by noise due to motion of the device with respect to the tissue and also by noise due to motion of the tissue itself (e.g., the expansion and contraction of tissue and vasculature), different noise may be captured by sensors corresponding to each of contacting and non-contacting light guides, and signal decomposition methods (e.g., blind source separation) may be employed to separate the signal of interest from the noise.
In Apple's patent FIG. 1 noted below we're able to see an electronic device (Apple Watch) having light sensors for determining a heart rate signal. A first light sensor #110 may be co-located with a contacting light guide #102 and a first light emitter. The contacting light guide may be configured so as to be in contact with tissue #114 of a user, such as skin.
For example, the contacting light guide may be curved such that the surface is configured to the contact tissue of the user. In some examples, the contacting light guide may jut out from the body of the electronic device (Apple Watch) such that it is configured to contact the tissue of the user.
A second light sensor #112 may be co-located with a non-contacting light guide #104 and a second light emitter #108. The non-contacting light guide may be configured so as to not be in contact with the tissue of the user. In some examples, the non-contacting light guide may be recessed with respect to the body of the electronic device such that it is configured not to contact the tissue of the user.
The electronic device (Apple Watch) may be situated such that the sensors #110 and #112, the emitters #106 and #108, and the light guides and are proximate the tissue of the user, so that light from a light emitter may be directed through a light guide and be incident on the tissue. For example, the electronic device 100 may be held in a user's hand or strapped to a user's wrist, among other possibilities.
A portion of the light from a light emitter may be absorbed by the skin, vasculature, and/or blood, among other possibilities, and a portion may be reflected back to a light sensor co-located with the light emitter. In some examples, light guides may direct light to tissue and/or back to a light sensor, and some emitters and sensors may direct light to and from tissue without a light guide.
In Apple's patent FIG. 2 noted below we're able to see a flowchart of a method of computing a heart rate signal. Light may be emitted from a first light emitter through a contacting light guide (#200 – top left box). First light information may be received at a first light sensor through the contacting light guide (#202). Similarly, light may be emitted from a second light emitter through a non contacting light guide (#204), and second light information may be received at a second light sensor through the non-contacting light guide (#206).
In some amount of light from a first light emitter that has been reflected by the skin, blood, and/or vasculature of the user, among other possibilities. In some examples, the first light information may indicate an amount of light from the first light emitter that has been absorbed by the skin, blood, and/or vasculature of the user.
For example, a light emitter may emit light, the light may travel to the tissue of a user, and a portion of the light may reflect to a co-located light sensor, and some or all of the travel of the light may be directed by a light guide. Accordingly, the first light information may indicate an amount of light from a first light emitter that has been reflected by the skin, blood, and/or vasculature of the user, among other possibilities. In some examples, the first light information may indicate an amount of light from the first light emitter that has been absorbed by the skin, blood, and/or vasculature of the user.
According to Apple, the light emitters may produce light in ranges corresponding to infrared (IR), green, amber, blue, and/or red light, among other possibilities. Additionally, the light sensors may be configured to sense light having certain wavelengths more easily than light having other wavelengths.
In some examples, a light emitter may be a light emitting diode (LED) and a light sensor may be a photodiode. The light information may include information produced by the photodiode. For example, the light information may include a voltage reading that corresponds to light absorbed by the photodiode. In some examples, the light information may include some transformation of raw signal produced by the photodiode, such as through filtering, scaling, or other signal processing. Those components may then be further processed with a cross-correlation method, serving as a second filter, to compute a heart rate signal.
When going to market with Apple Watch initially, Apple explained that they chose to use green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through the wrist at any given moment. When the user's heart beats, the blood flow in their wrist — and the green light absorption — is greater. Between beats, it's less. By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute — the user's heart rate. In addition, the heart rate sensor is designed to compensate for low signal levels by increasing both LED brightness and sampling rate.
Apple watch literature further states that "The heart rate sensor can also use infrared light. This mode is what Apple Watch uses when it measures your heart rate every 10 minutes." However, Apple later notes that "Apple Watch attempts to measure your heart rate every 10 minutes, but won't record it when you're in motion or your arm is moving."
This latest patent application filing from Apple sets out to compensate for any motion drawbacks interfering with the delivery of an accurate heart rate even while the user is in motion.
A Flowchart Method of Computing a Heart Rate
Apple's patent FIG. 3 noted below illustrates a method of computing a heart rate signal based on multiple wavelengths of contacting and non-contacting light information.
Within many of Apple's patents we'll find their engineers reveal ideas about the patent at hand that could apply to future products unintentionally. In this patent, the Apple engineer is talking about Touch I/O and states the following:
On Future Trackpads
The "Touch 1/0 device may include a touch sensing panel which is wholly or partially transparent, semitransparent, non-transparent, opaque or any combination thereof. The touch 1/0 device may be embodied as a touch screen, trackpad, a touch screen functioning as a trackpad (e.g., a touch screen replacing the trackpad of a laptop), a touch screen or trackpad combined or incorporated with any other input device (e.g., a touch screen or trackpad disposed on a keyboard) or any multi-dimensional object having a touch sensing surface for receiving touch input.
Having a touch screen as a trackpad on MacBooks would be interesting. Imagine being able to stay on top of your favorite sports team in action while you're working on a project at school or work while still having the trackpad fully functional. It would be like having a screen-within-a-screen trackpad. It could extend through to the Magic Trackpad for your iMac too. Of course there are many other possibilities you could come up with here with the trackpad doubling as a mini multitouch display.
If the Apple engineer is mentioning it generally in this patent filing, then it's likely something that the entire engineering team have been discussing over time as to where they could take the trackpad next. For now it's just an interesting future possibility.
Apple's patent application was published in Europe late yesterday. Apple credits Albert Wang, Optical Sensing Architect for this invention. Considering that this is a patent application, the timing of such a product to market is unknown at this time.
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